http://www.spridgetmania.com/ArticleV.cfm?DisplayID=1882

smaller round port heads with shared inlets making 150bhp
no reason why our better alloy round ports couldnt do better.

No problem. There are Datsun A12s that have made 150HP. Usually it is quite expensive.

Cheapest way perhaps is to turbocharge, intercool, use 115 octane petrol and rev to 10,000 rpm.

As usual when D says something, it is worth our attention.

The article says this:
Quote:

I like the fact that his engine does have to rev as high.

It appears he made the combustion chambers bigger too:

I should preface this slightly too long post with a quick statement: I'm not the worlds greatest expert on _anything_ . I've done a fair bit of stuffing around with other engines, which can be of some relevance. Anything I post is to the best of my knowledge, but I'm not perfect, and don't necessarily get it all right. I'll happily stand corrected on anything i post, and most of all I hope it comes across as a genuine attempt to share info, I'm certainly not knocking anyone else's contributions

I haven't read the article but I'd warn that there is a lot about mini heads that is very context specific. Taken out of context it can be very misleading.

In brief there are two types of mini engine - the large and small bore. The small bore blocks are all similar, but differing bore size, and the main contenders at the 998 and 1098 which share the same bore, but different stroke (the 1098 actually has the longest stroke of all mini engines). There were also inline a series engines, and the 1098s in inline configuration had same bore/stroke as the 1098 transverse engine, but the crank itself was much stronger (the inline and transverse cranks are not interchangeable, so ytou can put that stronger 1098 crank into minis even if you wanted to).

Then there is the 'big bore' mini engines. With 3 different strokes they came out as 970cc 1071cc and 1275cc. The 970 with 020 oversize or was it 030) came out at 999cc and 1cc under the 1 litre limit, with a big bore and short stroke (they had a slightly lower deck) they would rev their tits off. A couple of blokes in the late 60s were getting over 120bhp out of them - so more than 120bhp per litre, and revving to over 11,000rpm (same cams would only make power and result in approx 8500rpm upshifts with the larger 1275 engine and similar head)

Aside from the main bore differences between the two (and btw you can't put a 1098 crank in a 1275 block, but you can offset grind the 1275 crank and overbore to get beyond 1380cc) there was also bore spacing differences. the small bores had 2 bores close together - so 1-2-bigger gap-3-4 at the front and rear. The big bore blocks had evenly spaced bores - all about the same distance apart. This will get important later.

Basically there is a small port head for 850/998 engines. It's pretty much worthless. then there is the 12g202 head for 998 and 1098 engines Bigger ports to start with and more potential for porting to make power.

Then you have what is known as the 998 cooper head which came in numbers 12g295 (most common) and I think 12g206 (which was the same head but no grooves cut for alignment rings that lined up the intake manifold perfectly).

this 998 cooper head had bigger better flowing ports, larger valves, and a bathtub chamber (vs a sharp pointed heart shaped chamber for the other small bore heads). It was fitted to 998 coopers (the cooper S was always fitted with the large bore motors - 970 1071 and 1275, and larger disc brakes) and to morris 1100 (1098 engine) cars with automatic transmission (to make up for power loss of transmission) and 1098 inline engines.

Then there is the final head th 12g940. This was essentially a copy of the 998 cooper head, but it had slightly shallower chambers (998 engines required flat or even pop top pistons to get decent compression, whereas all engines with other than 998 cooper heads had slightly dished pistons to go with approx 21cc chambers) AND the chambers were relocated so as to suit the bore spacing of the large bore motors. Now the 998 cooper head is still 'the go' for the small bore engines. their small bore means valve shrouding is possible if you try and use large valves beyond minimal levels over stock.

These heads are harder to get. However, in some racing classes in the UK, and for those here who can't find a 998 cooper head that is usable, they use the 12g940 head. It's not so much that it 'kills' velocity - as you'll use all of the flow they can provide with a race engine (and a race engine designed for flexibility and a decently wide enough powerband too, not just some exotic narrow powerband nightmare to drive)) The BIG problem is that the exhaust valves will now open half way over the edge of the cylinder wall. So to get them to flow, and also to prevent the valve hitting the block at full lift, they have to make cuts into the top of the bores (considerable ones at that) to clear them. so it's a massive compromise to try and get that exhaust flowing right. It also then requires use of the big bore head gasket, which itself isn't as ideal for a smaller bore.

It never happened here (afaik) but in the UK there was a class of mini racing that used the 850cc engines with limited mods (entry level). with small engines and restricted cams and carbs/intake - they used the heart shaped chamber and did very minimal valve unshrouding, because at the arguably modest rpms the cam/carb could support and being the smallest engine, then more opening up of the chamber wouldn't up the flow enough to compensate for the power lost from lower compression from larger chamber volume. That's an extreme niche category and if they had scope to use a larger engine and better flowing head, those extreme small port/valve/heart shaped chamber heads and indeed the 850cc bottom end would never ever be used.

In one of David Vizard's mini/engine books, from approx the 70s he relates a fellow engine builder who took a mini head and angle milled/cut holes so that new intake ports made of exhaust tube could be pressed in on a 45 degree angle to meet up with teh bowl area of the factory port. It was effectively a semi downdraught head. With minimal bowl and valve guide/seat work beyond sealing it up, he dyno'd it and got over 150bhp (if I recall the figure) from a big bore combo - one that would have made perhaps 130 with the best conventional head of the time on the same engine.

If that weren't enough - much like the SBchevy - you can only open the beginning of the intake ports up to a certain size, as pushrods pass through the head on either side of the port, and if you open it too far, you break through. They sometimes open until just near that point, drill the pushrod hole a touch oversize, and press in a thinwall brass tube. Then you keep porting hte head until you just barely uncover the brass *(or even stainless perhaps). Point being, valve size is tied to port size (and of course general flow and shape of the port) and if there's a limit to the size of the port in one area, then the rest of the port won't likely gain from being a lot bigger than that, if anything it ends up creating an area of dead/no flow. So with this limitation, the port sizes etc that you see on mini heads are not indicative of ideal sizes, instead it's more like they are indicative of the best combination of compromises where you trade off as little as possible, but inevitably have to trade off a fair bit to make the 'best of a bad situation'

It's probably obvious to some that if 1 and 2 (and 3 and 4) share their intake ports, that when cam duration gets up there (esp the duration at more than 0.050" lift where flow 'really' starts happening) then there's an overlap - where #2 intake is closing, whilst #1 intake is starting to open. This overlap is enough that one cylinder tries to rob the other (and similarly as #3 intake is closing #4 is opening more and more). You can't just double port area, as that makes it way too big for the other 70% of each intake valves opening period, where it doesn't have to share. So this 'sharing' gets so bad that it costs power. The 'fix' is what is called a scatter pattern cam. They run the same duration on each intake, BUT they grind it so that intakes on 3 and 2 open approx 5 degrees sooner (which means they close 5 degrees sooner) than a technically ideal cam for that rpm range and #1 and #4 intake open and close 5 degrees later than ideal. This reduces the overlap and intake robbing by 10 degrees in total, and even though both have 'technically' wrong intake lobe centre lines, they consistently make more power, more torque and the powerband starts earlier and last longer. If you happen to want one of these cams, a lot of cam places will do them, but not all advertise it. From Wade, if you ask for a 176 you get the equivalent of the '649' factory/race cam. If you ask for the 176-0 they'll take the cam, weld up certain lobes and grind it in the scatter pattern. If you are going to do it, it's a good un.

If that wasn't bad enough, the centre exhaust port on a mini is shared by #2 ans #3 exhaust valves. Thankfully though, 2 and 3 are a full 360 degrees apart, so there's no overlapping or 'sharing' of the exhaust port - or more accurately they never try to both utilise that centre exhaust port at the same time, they alternate with a gap in between. To those unfamiliar with minis, the 3 primary pipe extractors would look a little weird at first glance, as if a pipe was missing, or it was for one side of a v6.

what I'm getting at is you have to do a hell of a lot of david copperfield like work to get mini engines/heads to work well, and not all of it is applicable to other engines. There _are_ some 8 port (and some 7 port,that use factory exhaust design, but new separate intakes) heads available for minis (they need cams to suit the revised valve layout which are rare or costly 1 offs) and you'll find that the sort of port sizing and valve sizing they go for is (or tries to be) much closer to the sort of thing we are more used to seeng on gx or other oval port heads. They are also a smaller engine (typically) than most datto a-series race engines, and even when similar size, typically smaller bore longer stroke, so they won't tend to work as well with the sort of port and valve sizes that a datto engine could and should have.

I'd also point out that minis (and it's partly due to the primitive port sharing etc) tend to like (for higher performance levels) tighter lobe separation angles by perhaps 1-3 degrees vs what the dattos tend to like so camshafts as well are difficult to compare.

Thanks for the kind words DD and Jmac for your lack of enthusiasm in mini engines :)

Im interested in the round port heads as the valve sizes and chamber shapes are very
similar to the best Mini A series engines. Im only looking at playing with 1300cc and
under so I feel the cheap heads are probably all I need.d

I was going to supercharge my a10 and a12 (vb10 and kb10) and was thinking I best use
the oval port heads as all the capacity in the ports I can allow will equal power.
However using the round ports cleaned up and following some of the chamber shapes that
are illustrated in that article seems like a good start.

It looks like the deep bathtub design is still kept but improvements made using less
shrouding around valves and removing the sharp edges for starters.
This depth allows the the mixture to be kept away from the cylinder walls and keep
the mix more central rather than spreading it out.

Looking at diesel engines of late they seem to focus the charge in the same manner or centred.
I think Jmac put it in the words "like a sphere" just before tdc as the best charge shape,
this sphere of suspended atomised fuel particles and air will make the best combustion.

Jmac excuse me for the not quoting your exact words as your posts are challenging :)

The problem with us carby bunch dinosaurs is that we dont have direct injection but we can
at least try to make this spherical charge effect by employing similar chamber designs
perfected by Coopers (not the Sth Oz beer company) in conjunction with concave pistons
like in the e15/16, some A14/15 and ga16 engines to help achieve the sphere charge.

interesting reading and once again, JMAc- longest post EVER!

i had a mini and put the 998 head on the 1098 block from memory (had a ex police mini deluxe fitted with cooper s spec 998 motor a nd box that went bang!)

dont forget the 1410cc Jmac too. the biggest badest modded 1275-s out there.

Are all Mini A series 3 bearing cranked?

yep

Hey D dont know if you remember but I posted some drawings of A series valve deshrouding based on some study of modified mini heads that I used the same principles and aplied to a series.

The bigest problem being that our a series head were allready much closer to the modified mini heads and there was little gain to be made without resesing valves witch then compromised port angle




edit: allso in the serch for allround power in the smaller capacities hase anyone ever 'filled' a GX head, were buy filling in the port on the bottom and raising it roof as high as posible creating a far mor gradual turn into the head? this might allso help with being able to reasess valves and regringing an optimum chamber?

edit edit: allso 'Tubeing' could be quite benificial especialy with the use of motobike carbs or fule injection. and I think it just about allways gets put in the to hard basket but they have all sorts of industrial vacume apoxi sealers these days that would make welding les crucial? just a thought..

I suppose it depends on which head we start from but for the early round ports Im sure there is
some cleaning up and slight deshrouding that could benefit flow.
It seems that the pic with the black has too much material removed unless the inlet valve is made larger as the shrouding is necessary to create the swirl on the sparkplug side of the chamber.
Maybe a tad of deshrouding woul benefit the higher end it just depends on what its tuned for.
I would lean towards as much swirl as possible with boost.

I dont think if tubing is worthwhile compared to a well matched port manifold.
Its easier to work on the ports the traditional way while other engines it could
work where there is enough meat to tap a thread into the ports to screw in a threaded
pipe of the right dimensions.
Im more inclined to think about using a unitary rocker cover top of similar grade alloy
plate then you could lift the port beyond sealing surface.
eg.

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